public class test08 {
    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode() {}
     *     TreeNode(int val) { this.val = val; }
     *     TreeNode(int val, TreeNode left, TreeNode right) {
     *         this.val = val;
     *         this.left = left;
     *         this.right = right;
     *     }
     * }
     */
        public int maxDepth(TreeNode root) {
            if(root==null){
                return 0;
            }
            int left = maxDepth(root.left);
            int right = maxDepth(root.right);
            return Math.max(left,right)+1;
        }
}
